Bonded sheet molding composite automotive components without surface pre-treatment

ABSTRACT

An automotive assembly comprises a sheet molding composite (SMC) element comprising an SMC sheet body shaped into a generally smooth functional section and at least one generally planar bonding section. The bonding section comprises a textured surface impressed as a result of molding the SMC sheet body in a compression molding tool having a corresponding textured pattern. The textured surface has a predetermined surface roughness for increasing the effective surface area of the bonding section relative to a non-textured surface. The predetermined surface roughness is in a range from 0.25 Ra (μm) to 8.5 Ra (μm). A second element has a secondary bonding surface adjoining the bonding section of the SMC element. An adhesive joins the secondary bonding surface to the bonding section without requiring any pre-treatment of the bonding section.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending U.S. application Ser. No.13/411,968, filed Mar. 5, 2012.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to automotive assembliescomprising sheet molding composites, and, more specifically, to improvedadhesive bonding of a sheet molding composite (SMC) element to otherelements.

Sheet molding composite (SMC) materials have been used and are beinginvestigated for making various kinds of structural automotivecomponents such as body panels (e.g., hoods, fenders, decklids, pickupboxes, pillars, lift gates, and roofs), engine components (e.g., valvecovers and oil pans), vehicle frame elements, bumper beams, fan shrouds,and many other types of components. The use of SMC materials has manyadvantages over steel and other types of composite material such ashand-laid fiberglass including lower weight, greater consolidation ofparts, ability to use less complex and expensive tooling for molding theparts, greater range of component styling, and short cycle times for themolding processes.

An SMC sheet or body is comprised of a sandwich formed with a resinpaste and chopped glass fibers between sheets of carrier films.Typically, one film is advanced under a box of resin paste which isdeposited onto the film. Glass fibers are threaded through a choppingmachine which cuts pieces of predetermined lengths and randomly depositsthem onto the advancing film and resin paste. A second film havingadditional resin paste deposited on it is laid atop the first film andcompacted into a sandwich that may be rolled up or folded for later usein a compression mold to form an SMC molded part.

A typical resin paste may be comprised of a filler (such as calciumcarbonate), polyester, a lower profile additive (such as polyvinylacetate or urethane), catalyst, a pigment, and other agents. Due to theinitial placement of the fibers at the center of the sandwich, theygenerally remain at the interior portions of a molded part away from thesurface.

A typical SMC element molded as part of an automotive assembly includesa functional section (e.g., for providing a Class A visible surface of apanel) and an attachment section for joining the SMC element to otherparts within an assembly. One desirable manner for joining an SMCelement to other elements is by adhesive bonding. In order to obtaingood adhesion, however, a bonding pad on the SMC element for receivingadhesive must usually be pre-treated because the smooth surface on atypical SMC bonding pad after compression molding lacks good adhesivecharacteristics.

Compression molding tools for SMC components typically employ highlypolished tool surfaces to achieve sufficient Class A surface quality.Chrome is usually deposited on the surfaces to achieve a high wearcapability during the molding process. Chrome helps resist the highlyabrasive nature of the SMC material during molding.

The smooth surface of an SMC bonding pad has been prepared for bondingby various pre-treatment methods known in the prior art, includingelectrical discharge treatment, plasma treatment, flame treatment, laseretching, gritblasting, sanding, and washing with solvents. Suchtreatments improve joint strength obtained by adhesive bonding. However,they result in undesirable costs, additional processing, andenvironmental consequences. It would be desirable to increase jointstrength without requiring any such pre-treatments.

SUMMARY OF THE INVENTION

In one aspect of the invention, an automotive assembly comprises a sheetmolding composite (SMC) element comprising an SMC sheet body shaped intoa generally smooth functional section and at least one generally planarbonding section. The bonding section comprises a textured surfaceimpressed as a result of molding the SMC sheet body in a compressionmolding tool having a corresponding textured pattern. The texturedsurface has a predetermined surface roughness for increasing theeffective surface area of the bonding section relative to a non-texturedsurface. The predetermined surface roughness is in a range from 0.25R_(a) (μm) to 8.5 R_(a) (μm). A second element has a secondary bondingsurface adjoining the bonding section of the SMC element. An adhesivejoins the secondary bonding surface to the bonding section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an SMC body.

FIGS. 2A and 2B are cross sections of an SMC body in a compression moldbefore and after compression, respectively.

FIGS. 3A and 3B are end views of a structural assembly wherein at leastone element of the assembly is formed from an SMC body and has a bondingsection with a textured surface according to the present invention.

FIG. 4 is a perspective view of an SMC structural element.

FIG. 5 is a flowchart showing conventional processing of an SMC elementprior to adhesive bonding.

FIG. 6 is a flowchart showing an improved method of the presentinvention for adhesively bonding an SMC element without conventionalpre-treatment.

FIG. 7 is a perspective view of a portion of a compression moldaccording to one embodiment of the present invention.

FIG. 8 is a plan view of one embodiment of a textured surface accordingto one embodiment of the present invention.

FIG. 9 is a plan view of one embodiment of a textured surface accordingto another embodiment of the present invention.

FIG. 10 is a cross section through an adhesive bond joining an SMCelement with a second element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, an SMC body 10 is comprised of a resin paste 11between a pair of films 12 and 13. A plurality of fibers 14 are embeddedin resin paste 11.

An SMC body 20 may be inserted into a cavity in a compression moldingtool having a lower portion 21 and an upper portion 22 configured toprovide a mold cavity in the shape of a desired final SMC element. Lowertooling die 21 has bonding pad portions 23 and 24 and upper tooling die22 has bonding pad sections 25 and 26, which are all conventionallygiven a smooth surface. Dies 21 and 22 are brought together to compressSMC body 20 while heat is applied in order to compression mold an SMCelement 28 as shown in FIG. 2B.

As shown in FIGS. 3A and 3B, SMC element 28 has bonding sections 30 and31 for bonding with a second element 32 in an automotive assembly.Bonding section 30 and 31 were conventionally pre-treated for betteradherence of an adhesive 33 for joining elements 28 and 32 as shown inFIG. 3B.

FIG. 4 shows bonding sections 30 and 31 on SMC element 28 intended toreceive the adhesive. Bonding surfaces 30 and 31 are compression moldedand, therefore, had a smooth surface in the prior art due to a desire toprovide a long lifetime for the compression molding die tools thatinterface with the SMC material. In order to deal with the abrasivetendencies of the SMC material, and due to the desire to obtain asmooth, blemish-free surface on all Class A portions of compressionmolded elements, all tooling surfaces have conventionally been polishedsmooth and then coated with chrome. Due to the smooth surfaces ofbonding sections 30 and 31, pretreatment was necessary when usingadhesive bonding.

One conventional prior art manufacturing method is shown in FIG. 5wherein an SMC part is compression molded with a bonding pad in step 40.The bonding pad on the SMC part is then pre-treated in step 41 in orderto roughen or activate the surface. In step 42, the SMC part and asecond part to which it is being bonded are placed in a bonding fixture.Adhesive is applied in step 43, and the parts are compressed and theadhesive is allowed to cure in step 44. The surface treatment such asscuff sanding, flame treatment, and plasma treatment that have beenroutinely required to achieve acceptable bond performance were timeconsuming, labor intensive, and resulted in additional costs for themanufacturing of SMC components.

The present invention enhances SMC bonding by incorporating a texturedtool surface for the bonding pads to obtain superior bonding adhesionwithout the need for pre-treatments. By deliberately introducing apredetermined surface roughness created by the mold at the bonding padareas, an increased area of the bond surface is created which in turnprovides increased joint strength. In one embodiment of a method of thepresent invention shown in FIG. 6, a texture is applied to a moldtooling die at the bonding pad surface in step 45. An SMC part iscompression molded in step 46. In step 47, adhesive is applied to thebonding pad without pre-treatment. Thereafter, the parts to be bondedare compressed and the adhesive is cured in a conventional manner. Thesecond component can be made of any desired material including a secondSMC element with bonding pads similarly textured for ensuring excellentjoint strength.

FIG. 7 shows a tooling die 51 having bonding pad portions 52 and 53 anda functional portion 54. Portions 52 and 53 have a predetermined surfaceroughness for increasing the effective surface area relative to anon-textured surface. Preferably, the predetermined surface roughness isin a range from 0.25 R_(a) (μm) to 8.5 R_(a) (μm). Textured surfaces 52and 53 impress a corresponding texture into the SMC sheet body duringcompression molding so that the corresponding generally planar bondingsections on the SMC element are produced with the desired surfaceroughness, leading to enhanced joint strength when adhesively bonded.

Various textures can be employed to achieve the desired surfaceroughness. In one example, a multiplicity of lineal “scratches” isemployed as shown in FIG. 8. Such scratches can be formed on the moldingdie by etching or machining to create a surface similar to what wouldresult from scratching with sandpaper of various grits as reflected inthe following table.

Sample Grit Direction R_(a) (μm) 1 400 Parallel 0.35 2 400 Transverse0.25 3 240 Parallel 0.84 4 240 Transverse 0.72 5 120 Parallel 1.47 6 120Transverse 1.48 7 80 Parallel 3.6 8 80 Transverse 5.64Direction refers to whether the roughness measurement (e.g., using aprofilometer) is taken in a direction parallel or transverse with thedirection of the “scratches”. Samples 1 and 2 correspond to altering ofthe tooling surface in a manner equivalent to a 400 grit sandpaper in adirection parallel and transverse to the measurement direction,respectively. The resulting surface roughness for die samples 1 and 2produce a value of R_(a) of 0.35 and 0.25, respectively. As known in theart, R_(a) is the arithmetic average of vertical deviations in aroughness profile.

In samples 3 and 4, a texture corresponding to a 240 grit sandpaperproduces roughness R_(a) with values of 0.84 and 0.72, respectively.Altering the tooling surface equivalent to sandpaper grits of 120 and 80result in the surface roughness as shown in the table. Using a texturethat is coarser than 80 may be undesirable because the correspondingsurface roughness would tend to expose internal fibers. A finer texturethan 400 would be undesirable since insufficient surface roughness wouldbe created and no significant increase in mechanical joint strengthwould be obtained. The most preferable range in predetermined surfaceroughness lies from about 0.35 R_(a) to about 3.6 R_(a) (μm).

FIG. 9 shows an alternative texture wherein any common stipple patternwith a grain exhibiting the desired surface roughness may be employed.Any stipple pattern could be selected provided that the roughness R_(a)of the stipple pattern falls in the range from 0.25 R_(a) (μm) to 8.5R_(a) (μm).

FIG. 10 shows a first SMC element 55 and a second SMC element 56 withtextured surfaces 57 and 58 having the predetermined surface roughness.An adhesive 60 has been applied and cured between textured surfaces 57and 58. The respective surface areas of each SMC element that are incontact with adhesive 60 are increased relative to a non-texturedsurface to thereby increase joint strength. The surface texture isapplied in a manner that avoids disruption of the internal fibers whichcould otherwise lead to delamination if fibers extended out from the SMCsurface.

What is claimed is:
 1. A method of making an automotive assembly, comprising the steps of: preparing a compression mold having a smooth functional portion and a bonding pad portion, wherein the compression mold has a textured surface defining the bonding pad portion, wherein the textured surface has a textured pattern of generally parallel scratches to provide a predetermined surface roughness for increasing the effective surface area of the bonding pad portion relative to a non-textured surface, wherein the predetermined surface roughness is in a range from 0.35 R_(a) (μm) to 3.6 R_(a) (μm) in a direction parallel with the scratches and is in a range from 0.25 R_(a) (μm) to 5.64 R_(a) (μm) in a direction transverse with the scratches, and wherein the smooth functional section has a respective surface roughness less than 0.25 R_(a) (μm); inserting an SMC sheet body into the compression mold to shape the SMC sheet body into a generally smooth functional section within the functional portion of the compression mold and at least one generally planar bonding section within the bonding pad portion of the compression mold; removing the shaped SMC sheet body from the compression mold; aligning the bonding section with a second assembly element having a secondary bonding surface; and bonding the bonding section and the secondary bonding surface with an adhesive.
 2. The method of claim 1 wherein the second assembly element is made by the method further comprising the steps of: preparing a second compression mold having a second functional portion and a second bonding pad portion, wherein the second compression mold has a second textured surface defining the second bonding pad portion, and wherein the second textured surface has a second textured pattern of generally parallel scratches to provide a second predetermined surface roughness for increasing the effective surface area of the second bonding pad portion relative to a non-textured surface, wherein the second predetermined surface roughness is in a range from 0.35 R_(a) (μm) to 3.6 R_(a) (μm) in a direction parallel with the scratches and is in a range from 0.25 R_(a) (μm) to 5.64 R_(a) (μm) in a direction transverse with the scratches, and wherein the second functional portion has a respective surface roughness less than 0.25 R_(a) (μm); inserting a second SMC sheet body into the second compression mold to shape the second SMC sheet body into a second generally smooth functional section within the second functional portion of the second compression mold and into the secondary bonding surface within the second bonding pad portion of the second compression mold. 